Australia needs a backup plan for the Aukus submarine agreement, Labor MP Ed Husic has warned, arguing sluggish American production and the “transactional nature” of the Trump administration have put the multi-billion-dollar defence deal at risk.

The defence minister, Richard Marles, this week agreed to US requests for Australia to accept three second-hand Virginia-class nuclear submarines, rather than a combination of new and old vessels.

Husic spoke out during a closed-door meeting of the Labor caucus on Tuesday.

It was the most significant internal criticism of the $368bn deal – agreed by the Morrison government in 2021 and endorsed by the then-Labor opposition – since heated debate at the ALP national conference three years ago. Labor ultimately continued its support of the multi-decade pact.

Husic said production rates of submarines in the US were too low for Australia to realistically expect boats to be handed over in the early 2030s.

The deal requires the sitting US president to agree to release submarines based on the US having an adequate supply for its own navy, even though Australia is paying to boost production.

“We need to be open as a nation that we are not going to get the deal that was promised to us,” Husic said.

“Given how transactional the Trump administration is, you can almost imagine them saying ‘we give you these, you will do this with them’, and so there’s an active sovereignty question there.

“It won’t be a renegotiation; it’s a reality about the production rates and whether or not we’ll get them. What’s the contingency? What’s the plan B?”

US shipyards currently produce between 1.1 and 1.2 Virginia-class submarines each year, well below the target yearly rate of 2.33 needed for the deal to go ahead as planned.

Husic said there was disquiet about Aukus within the wider party rank and file. He suggested Marles had been forced by the US to say he was happy about the new arrangements after weekend talks with his counterpart, Pete Hegseth, in Singapore.

“There’s an issue about [the] reality … confronting us, about whether or not we will even get the new deal that has been put to us based on what’s happening in the US,” Husic said.

The former cabinet minister was dumped in a factional deal orchestrated by Marles after the 2025 election. He is close to the former prime minister Paul Keating, one of the loudest critics of the Aukus plan.

The shadow defence minister, James Paterson, said Husic’s intervention represented a “full-on Labor revolt”. Paterson demanded Marles pull his colleague into line and reaffirm the government’s commitment to Aukus.

“It’s absolutely legitimate to ask questions about how this government is going about delivering Aukus, about the details of Aukus,” the Liberal MP said.

“What is much more concerning is to have a former cabinet minister still in the Labor caucus questioning the merits of Aukus altogether.”

Paterson questioned why secondhand submarines would be cheaper and easier to operate, as claimed by Marles. “If that was the case, why wasn’t this the optimal pathway three years ago,” he said.

After meetings on the sidelines of the Shangri-La Dialogue on Sunday, Marles said servicing and training efforts would be streamlined because Australian crews would not operate two different American-made submarines before the bespoke SSN Aukus model comes online in 2042.

The first Virginia-class from the US was due to arrive in Australia in 2032, with another arriving every four years, before the Australian-built model was ready for operations.

Husic’s comments come on the same day former Labor minister Peter Garrett was announced as the head of a public inquiry into Aukus, backed by unions and non-profit groups.

Former West Australian Labor premier Carmen Lawrence and former defence force chief Chris Barrie were among the commissioners chosen for the inquiry, which is not a parliamentary review.

Garrett said there had been no proper parliamentary scrutiny of the deal, calling it “the most momentous and expensive decision ever made by any Australian government in the modern era.”

Opposition to Aukus continues to harden in the wider Labor movement, setting the stage for an internal fight at the party’s upcoming national conference in Adelaide.

A motion calling on the Albanese government to review the security pact last month won support in the Victorian branch for the second year in a row.

The grassroots Labor Against War action group is pushing to strip all references to Aukus from the national platform, which is up for debate at the July conference.

The group’s submission to consultation on the draft platform, seen by Guardian Australia, seeks to insert a reference to the “illegal US-Israel war on Iran” and guarantee that Labor would not commit military forces to an armed conflict that is “not consistent with international law”.

The treasurer, Jim Chalmers, said on Tuesday that Labor remained committed to delivering Aukus.

Arthur Rorris, the secretary of the South Coast Labour Council, which opposes the establishment of a nuclear submarine base at Port Kembla, said the proposed base was never intended for Australian submarines but “would be ceded to the United States navy as a staging post for their 7th fleet”.

“Building submarines for Australia was never at the top of Washington’s agenda; establishing a base for their forever wars certainly is,” Rorris said on the weekend.

– Additional reporting by Ben Doherty